This proposal is concerned with obtaining some understanding of the early physicochemical events involved in radiation effects in cells. The approach employed based on experiments using a field emission accelerator as a radiation source is to measure the kinetics and temperature dependence associated with changes of radiation sensitivity in untreated cells. The radiation sensitivity can them be manipulated by the addition of various agents such as sensitizers, protectors or D2O. Throughthe measurement of the kinetics and temperature dependence of the treated cells it is possible then to determine not only the effect n the early events but to gain knowledge of the action of the agents. Spores, bacteria and mammalian cells will be used. Experiments are also proposed to examine the influence of LET on the temperature and water dependence of radiosensitivity through irradiation of bacteria or spores with particles from a cyclotron. These studies are expected to yield some understanding of how radiation kills cells and whether knowledge gained with one type of cell is applicable to another. This question has importance for the prediction of effects of find ways to treat or maximize treatment of cells by radiotherapy.